The present invention concerns the field of frequency modulation receivers (hereinafter xe2x80x9cFM receiversxe2x80x9d) able to receive a signal at a high frequency from a transmitter, particularly for a Radio Data System (RDS) application.
With reference to FIG. 1 of the present description, French Patent No. 2705,176 discloses a conventional FM receiver 1 which includes an antenna 2, a high frequency stage (RF) 3, a mixer 4, a local oscillator 5, an intermediate frequency (IF) filter 6, an amplifier/limiter 7, an FM demodulation stage 8 and an automatic frequency control (AFC) stage 9.
Antenna 2 receives a signal having a frequency comprised within the FM transmission band (i.e. between 88 and 108 MHz), and supplies the corresponding electric signal to RF stage 3.
RF stage 3 is tuned onto the carrier frequency fc of a predetermined transmitter so as to provide, in response to the electric signal originating from the antenna, an amplified signal having a frequency comprised within a frequency band centred on frequency fc.
Mixer 4 receives the signal originating from RF stage 3, as well as a signal originating from local oscillator 5, multiplies these signals, and supplies to IF filter 6 a signal modulated at an intermediate frequency fIF, the latter being generally chosen to be equal to approximately 70 kHz.
IF filter 6 is arranged to receive the signal modulated at frequency fIF and, in response to provide a signal located within a frequency band centred around intermediate frequency FIF.
Amplifier/limiter 7 receives the signal originating from IF filter 6, limits its amplitude to eliminate any amplitude modulation (AM) component and, in response, provides this amplified/limited signal.
FM demodulation stage 8 demodulates the signal originating from amplifier/limiter 7, and provides the demodulated signal to an audio amplifier (not shown in FIG. 1).
AFC stage 9 allows fine tuning to be performed between frequency fLO of the signal from local oscillator 5 as a function of the continuous output of demodulator 8 in order to keep intermediate frequency fIF constant.
One problem of an FM radio receiver such as that shown in FIG. 1 consists in the fact that considerable distortion can appear by aliasing, when the modulation frequency (i.e. that of the message signal) is greater than half of intermediate frequency fIF.
The problem of aliasing distortion arises particularly in transmission systems using frequency division multiplexing, as is the case in Europe with FM broadcasting, in particular for an RDS application. Indeed, the effect of such distortion can be that decoding the RDS data becomes impossible.
FIG. 2 of the present description shows the frequency spectral distribution of the signals present in FM receiver 1, during an RDS application. Audio data are modulated in stereophony on a sub-carrier of 38 kHz, while other data corresponding to the RDS data are present in the form of low amplitude signals modulated on a sub-carrier of 57 kHz by phase shift (PSK modulation). In order to allow the RDS data to be decoded, and with reference once more to FIG. 1, the output bandwidth of FM demodulation stage 8 would have to be at least 60 kHz. Indeed, assuming that the intermediate frequency fIF used is of the order of 70 kHz, the audio data and the RDS data can be mixed by aliasing.
A first conventional solution to the problem of frequency aliasing consists in using a higher frequency fIF. U.S. Pat. No. 4,885,802 discloses an FM receiver implementing such a solution.
One drawback of such an FM receiver lies in the fact that it requires the use of additional means to filter the response of the image frequency of the RF signal, which also increases the electric power consumption.
Another drawback of such an FM receiver lies in the fact that the maximum intermediate frequency which can be used is imposed by the image frequency rejection and by the spacing of the channels or transmitters in the FM broadcasting band. Consequently, the audio bandwidth of the demodulated signals cannot thus simply be improved by increasing intermediate frequency fIF of the FM receiver.
A second conventional solution to the problem of frequency aliasing consists in providing a signal having a higher frequency than intermediate frequency fIF to the demodulation stage.
With reference to FIG. 3 of the present description, U.S. Pat. No. 5,483,695 discloses an FM receiver implementing such a solution. It will be noted in FIG. 3 that similar objects to those in FIG. 1 have been designated by the same references. FM receiver 20 further includes a signal generator circuit 24 able to generate n signals at intermediate frequency fIF, and a multiplier circuit 28 able to multiplier between them the n signals to provide a signal modulated at a frequency equal to n times intermediate frequency fIF.
One drawback of FM receiver 20 lies in the fact that it is necessary to generate signals in phase quadrature and having a frequency equal to intermediate frequency fIF, which considerably increases the electric power consumption.
One object of the present invention is to provide an FM radio receiver for an RDS application which overcomes the aliasing problem, in particular an FM radio receiver able to extract the RDS data present in a radio-frequency signal received by said receiver.
Another object of the present invention is to provide an FM radio receiver answering the usual criteria in the semiconductor industry as to low electric power consumption.
Another object of the present invention is to provide an FM radio receiver in the form of an integrated circuit answering the usual criteria in the semiconductor industry as to rationality and surface occupation.
These objects, in addition to others, are achieved by the FM receiver according to claim 1.
One advantage of the arrangement of the locked loop of such a receiver is that intermediate frequency fIF is enslaved to a predetermined value so that the aliasing phenomenon does not prevent decoding of the RDS data present in a radio-frequency signal received by the receiver.
Another advantage of such an arrangement is that the receiver may operate with low intermediate frequencies fIF, without it being necessary to use high frequency values, or frequency multiplication. This results in low electric power consumption, which allows an FM receiver of this type to be used for an RDS application, for example.